1. Field of the Invention
The present invention relates to a filter circuit and laminate filter used in a high-frequency range and, more particularly, to a filter circuit and laminate filter having attenuation bands on both low- and high-frequency sides.
2. Description of the Related Art
The principle of a conventional stripline filter is as follows. A stripline is disposed on a dielectric layer. One end of the stripline is short-circuited, the other end being open. This stripline filter adopts either an electric field-coupled type producing stronger electric field coupling or a magnetic field-coupled type producing stronger magnetic field coupling according to arrangement of resonators or by addition of capacitively coupled electrodes or the like. In the case of a filter in which the electric field coupling is stronger, there is a tendency of low-frequency attenuation. On the other hand, in the case of a filter in which the magnetic field coupling is stronger, there is a tendency of high-frequency attenuation.
Techniques disclosed in JP-A-H8-23205 (well-known example 1), JP-A-2002-26607 (well-known example 2), and JP-A-2002-76705 (well-known example 3) are examples of conventional techniques.
The fundamental embodiment disclosed in the well-known example 1 in the aforementioned prior-art examples comprises a first dielectric substrate 2 on which resonant electrodes 12a and 12b are formed, a second dielectric substrate 4 on which an internal grounding electrode 22 is formed, a third dielectric substrate 6 on which an external grounding electrode 16 is formed, and a fourth dielectric substrate 8 on which a capacitively coupled electrode 140 is formed, as shown in FIG. 1 of the reference. The degree of coupling is enhanced by an M-coupled electrode that is the internal grounding electrode 22 so as to adjust the frequency characteristics. An attenuation pole is formed by the capacitively coupled electrode. In this well-known example 1, the attenuation pole exists only in a low-frequency range as disclosed in FIG. 7 of the reference.
The fundamental embodiment disclosed in the well-known example 2 in the aforementioned examples is shown in FIG. 3 of the reference that is a virtual perspective view of the lamination of dielectric substrates 1c and 1d. In FIG. 3, the center-to-center spacing between resonator electrodes 11a and 11b is made coincident with the center-to-center spacing between notched capacitive electrodes 4a and 4b. In this way, when the amount of electromagnetic field coupling is controlled, it can be controlled by varying the length of a shared electrode portion 12 without changing the spacing. That is, the attenuation pole disclosed in FIG. 8 of the well-known example 2 is formed by the notched capacitive electrodes 4a and 4b. The stop band is controlled by varying the length of the shared electrode portion 12. In this well-known example 2, the attenuation pole exists only in a high-frequency range.
The fundamental embodiment disclosed in the well-known example 3 in the aforementioned well-known examples is shown in FIG. 2 of the reference. That is, dielectric layers 4a-4d are stacked. An upper electrode 5b is formed on the surface of the dielectric layer 4a. An end-surface electrode 5c is formed on the rear surface of the dielectric layer 4d. Striplines 1a and 1b are formed on the surface of the dielectric layer 4c. A shorting electrode 10 is formed in which one end of the each striplines 1a and 1b is connected substantially with the whole region of the end-surface electrode 5c. A stray capacitance electrode 9 is formed on the surface of the dielectric layer 4b perpendicularly to the striplines 1a and 1b. The attenuation band is adjusted by the stray capacitance electrode 9. The width of the high-frequency band is adjusted by the shorting electrode 5c that is M-coupled. Also, in this well-known example 3, the attenuation band exists only in a high-frequency range.
In any of the aforementioned well-known examples, both C-coupled and M-coupled patterns are provided to control the attenuation band. In these well-known examples, the controllable attenuation band is only on the low-frequency side (well-known example 1) or only on the high-frequency side (well-known examples 2 and 3).
Under circumstances where communication devices such as mobile phones are required to be diversified, laminate filters are required to have attenuation-band characteristics that are steep on both low- and high-frequency sides. In the conventional laminate filters, an attenuation band is formed only on the low-frequency side or high-frequency side as described above.